Most oil and gas wells are drilled with a rotary drilling rig. Typically, the drill string has a drill bit on the end and is rotated to cause the drill bit to advance into the earth. A drilling fluid is pumped down the interior passage of the drill pipe, which exits nozzles on the drill bit and flows back up an annular space surrounding the drill pipe along with cuttings.
Normally, the drilling fluid is a liquid called mud, which has a weight selected to provide a hydrostatic pressure greater than the expected earth formation pressures. When tripping the drill string in and out of the hole, the drilling mud in the hole and within the interior of the drill pipe provide sufficient hydrostatic pressure to prevent a blowout. However, heavy drilling mud can damage certain earth formations, reducing their abilities to produce fluids after completion. For example, methane is located in certain fairly deep coal beds. The coal formations may be damaged by encroaching drilling mud.
Drilling with gaseous fluids, such as air, has also been done with oil and gas wells. In one techniques, compressed air flows down the interior of the drill pipe, exits the drill bit and flows back up the annulus. A stripper seal surrounds the drill pipe at the surface for sealing the gas pressure in the well. Also, compressed air is used as a drilling fluid for drilling shallow mining blast holes. Mining drilling rigs may employ a dual passage string of drill pipe, with one of the passages being an inner passage and the other an annular passage. A gaseous fluid such as air is pumped down the annular passage and flows back up the inner passage along with cuttings. The dual passage drill pipe can be rotated to rotate the drill bit. Alternately, a downhole motor can be utilized which may also create a reciprocating a hammer motion as well as rotating the drill bit while the drill pipe remains stationary.
The possibility of a blowout due to excessive earth formation pressure is not a factor with shallow drilling of mining blast holes. With deep oil and gas drilling, however, it must be considered both while drilling and while tripping the drill pipe in and out of the hole. Blowout preventers and rams are utilized to seal around the annulus of drill pipe. The use of check valves in the drill string has been proposed in the past. The primary barrier to a blowout, however, continues to be the use of drilling mud with sufficient weight to provide a higher hydrostatic pressure than any expected pressure of the earth formations.